Conventionally, as a development method in which a non-magnetic one component developer is used as a developer (toner), known is a development method (impression development) in which a toner is provided via a developing roller on an image holding body such as a photoconductor drum on which an electrostatic latent image is held, and the toner is attached to the latent image on the image holding body, thereby visualizing the latent image. Since, in this development method, development is performed by adhering a toner on the latent image on the image holding body by contacting the developing roller on which a toner is held on an image holding body which holds an electrostatic latent image, the developing roller used for the development method needs to be formed of an elastic body having electrical conductivity.
FIG. 2 illustrates one example of the structure of a development apparatus using impression development. In the illustrated development apparatus, a developing roller 10 is arranged between a toner supplying roller 11 which supplies toner and a photoconductor drum 12 which holds an electrostatic latent image in a state in which the developing roller 10 is in contact with the photoconductor drum 12. Each of these rollers, the developing roller 10, the photoconductor drum 12 and the toner supplying roller 11 rotates in the arrow direction in the figure, and toner 13 is supplied on the surface of the developing roller 10 by the toner supplying roller 11. The supplied toner is adjusted in an uniform thin layer by a layer forming blade 14. The developing roller 10 in this state rotates while being in contact with the photoconductor drum 12, and thus the toner formed in a thin layer adheres to the latent image on the photoconductor drum 12 from the developing roller 10, whereby the latent image is visualized. Reference numeral 15 in the figure indicates a transfer unit, where a toner image is transferred to recording media such as a paper. Reference numeral 16 indicates a cleaning unit, by which toner remaining on the surface of the photoconductor drum 12 after the image is transferred is removed by a cleaning blade 17.
As an improvement technique relating to an electrically conductive roller, for example, Patent Document 1 discloses an electrically conductive roller for an electrophotographic device in which an electrically conductive elastic layer is composed of a polyurethane obtained by using a polyol whose hydrophobicity is larger than that of polypropylene ether glycol. Patent document 2 discloses a developing roller for an electrophotographic device in which a polyurethane elastic body formed by using a mixed polyol obtained by mixing a polypropylene polyol and a hydrophobic polyol and a polyisocyanate is arranged around a cored bar. Further, Patent Document 3 discloses a roll member for an electrophotographic machine which uses foamable polymer composition containing as essential components (A) a polymer whose main chain is composed of a structural unit derived from a saturated hydrocarbon polymer and which has at least two hydroxyl groups, (B) a hydrosilylated compound and (C) a catalyst.
By the way, when a white solid image is printed under high-temperature and high-humidity environment, at a temperature of about 32.5° C. and at a humidity of about 80% RH, the above-mentioned apparatus using the development method sometimes causes so-called fogging in which toner held on the surface of a developing roller adheres to a portion which is to be white background on an image holding body and the toner is transferred to a white background portion on a printed image. One of the causes of such occurrence of fogging under high-temperature and high-humidity environment is insufficiently charged toner. This is because, under high-temperature and high-humidity environment, there is small static electricity and electrostatic adhesion of the toner to the developing roller becomes weak. For this reason, studies have conventionally been made in order to improve the toner electrification amount as a measure to prevent fogging under high-temperature and high-humidity environment.
On the other hand, to provide a developing roller on the surface of which the occurrence of filming is suppressed, and which forms a high-quality toner image having high density and free from fogging, and does not cause soiling in a device, for example, Patent Document 4 discloses a technique in which, in a developing roller in which a coating layer having conductivity is formed on the outer periphery of an electrically conductive shaft, when a resin thin film composed of only resin which forms the coating layer is electrified by an electrifier, letting the surface potential of the resin thin film one second thereafter be Rv1 and the surface potential of the resin thin film ten seconds thereafter be Rv2, the developing roller satisfies the relation: 0.01≦Rv2/Rv1≦0.20.
It is known that, in the case that the developing roller has a constitution composed of three layers, an elastic layer, an intermediate layer and a surface layer each successively formed on the outer periphery of a shaft, by designing the resistance of the intermediate layer to be low, change in resistance value due to environmental variation can be made small, thereby improving the printing quality. For this reason, carbon black is generally added to the intermediate layer in order to reduce the resistance value of the intermediate layer. On the other hand, as a resin component constituting the intermediate layer, a thermosetting resin or a thermoplastic resin has been conventionally used. The thermosetting resin, for example, needs a large amount of heat and much time for curing, which has not been satisfactory in view of environmental friendliness or production efficiency.
On the other hand, for example, Patent Document 5 discloses an electrically conductive roller in which at least one layer of one or more elastic layers arranged in the radial direction outside of a shaft member is constituted by an ultraviolet curable resin containing a conductive agent and an ultraviolet polymerization initiator. In Patent Document 5, in an Example in which carbon black is used as a conductive agent of the elastic layer, the elastic layer is cured by irradiation of not a UV light but an electron beam (EB). Patent Document 6 discloses a developing roller comprising a shaft, a non-foam elastic layer formed on the outer periphery of the shaft and at least one layer of resin coating layers formed on the outer periphery of the non-foam elastic layer, the resin coating layer being composed of an ultraviolet curable resin containing a conductive agent.